On September 10th Twitter released a new Terms of Service (ToS) that you as a user of Twitter should be aware of. Some of the changes related to privacy and security are noted below with my comments in bold:

The Content you submit, post, or display will be able to be viewed by other users of the Services and through third party services and websites.
This should be obvious but by using Twitter you should have no expectation of privacy at all (even with a “private” profile).

In consideration for Twitter granting you access to and use of the Services, you agree that Twitter and its third party providers and partners may place such advertising on the Services or in connection with the display of Content or information from the Services whether submitted by you or others.
Twitter has to make money somehow so don’t be shocked when you see ad’s being generated based on the content of your tweets.

You are responsible for safeguarding the password that you use to access the Services and for any activities or actions under your password. We encourage you to use “strong” passwords (passwords that use a combination of upper and lower case letters, numbers and symbols) with your account. Twitter cannot and will not be liable for any loss or damage arising from your failure to comply with the above requirements.
This shouldn’t be a surprise either. If your password gets owned by a hacker, Twitter is not responsible. However, I still think that Twitter should require stronger passwords on their end.

You understand that by using the Services, you may be exposed to Content that might be offensive, harmful, inaccurate or otherwise inappropriate, or in some cases, postings that have been mislabeled or are otherwise deceptive.
Disinformation is a popular tactic on Twitter used by spammers as well as people that want to spread incorrect information about news and other topics. Twitter is not responsible for this type of behavior. You don’t believe *everything* you read on Twitter right?

By submitting, posting or displaying Content on or through the Services, you grant us a worldwide, non-exclusive, royalty-free license (with the right to sublicense) to use, copy, reproduce, process, adapt, modify, publish, transmit, display and distribute such Content in any and all media or distribution methods (now known or later developed).Sure, the content you post is yours but whatever you post can be modified, retransmitted, etc by Twitter and third-party apps that interact with Twitter.

…you have to use the Twitter API if you want to reproduce, modify, create derivative works, distribute, sell, transfer, publicly display, publicly perform, transmit, or otherwise use the Content or Services.
This is the reason that the Twitter API is so open and also the primary reason that spammers and other people with bad intent can take advantage of the service.

You may not do any of the following while accessing or using the Services: (i) access, tamper with, or use non-public areas of the Services, Twitter’s computer systems, or the technical delivery systems of Twitter’s providers; (ii) probe, scan, or test the vulnerability of any system or network or breach or circumvent any security or authentication measures…This is interesting to me. So if you are a security researcher you cannot “test” Twitter for vulnerabilities. That would include fuzzing and/or doing simple tests for XSS. So if you find a vulnerability on Twitter and disclose it to them can they delete your account, or report you to law enforcement? Remember kids…don’t test for vulnerabilities without permission first.

…or (v) interfere with, or disrupt, (or attempt to do so), the access of any user, host or network, including, without limitation, sending a virus, overloading, flooding, spamming, mail-bombing the Services, or by scripting the creation of Content in such a manner as to interfere with or create an undue burden on the Services.The part about flooding and mail-bombing the Services relates to the recent Twitter DD0S I suspect.

Twitter will not be responsible or liable for any harm to your computer system, loss of data, or other harm that results from your access to or use of the Services, or any Content. You also agree that Twitter has no responsibility or liability for the deletion of, or the failure to store or to transmit, any Content and other communications maintained by the Services. We make no warranty that the Services will meet your requirements or be available on an uninterrupted, secure, or error-free basis.If you use Twitter (or any social network for that matter) don’t assume that it’s “secure”. They don’t guarantee security an you shouldn’t either. Also, if you see the Fail Whale…it’s also not guarantee of service availability.

These are the main changes that I picked out related to privacy and security. However, you should really read the full ToS as it has gotten more detailed then the previous version. I would suspect more communication from Twitter on future changes to the ToS.